THURSDAY, Nov. 17 (HealthDay News) -- The 19 million new cases of syphilis, gonorrhea and chlamydia diagnosed in the United States each year cost the nation's health care system $17 billion annually, according to an annual report released Thursday by the U.S. Centers for Disease Control and Prevention.

Most sexually transmitted diseases (STDs) are treatable but can cause serious, life-long consequences, such as infertility, if they aren't detected.

"STDs are one of the most critical health challenges facing the nation today," CDC researchers said in their report.

Reported cases of chlamydia steadily increased for the past 20 years and reached 1.3 million in 2010. The increase stems from expanded screening efforts, not an actual rise in the number of people infected with chlamydia.

However, a majority of chlamydia infections still go undiagnosed, and fewer than half of sexually active young women undergo annual screening as recommended by the CDC.

Rates of gonorrhea are at historic lows, but more than 300,000 cases were diagnosed in 2010. There are also indications that the disease may be developing resistance to the only available treatment option, according to the CDC.

The syphilis rate fell 1.6% from 2009 to 2010, its first decrease in a decade. But the rate among young black men rose 134% since 2006.

Syphilis has also increased significantly among young, black gay and bisexual men, which suggests that new infections in this group are fueling the overall rise in syphilis infections among young black men.

This is particularly concerning because there has also been a sharp increase in HIV infections in the black gay and bisexual population, the CDC said.

The report also noted ongoing health inequalities linked with STDs. Blacks and Hispanics are more affected by STDs than whites. This is because many of the same social and economic factors -- such as low income and lack of access to health care -- that place blacks and Hispanics at higher risk for other diseases, such as diabetes and heart disease, also increase their risk for STDs.

In addition, young people represent 25% of people with sexual experience in the United States, but account for nearly half of new STDs, the CDC said.

While doctors must report cases of gonorrhea, chlamydia, and syphilis to local or state health departments, other STDs, including human papillomavirus (HPV) and genital herpes, are not included in the reporting system. Because of this, the true incidence of STDs is underestimated, the CDC said.